Bow thruster friction reduction

I have seen some yachts equipped with semi circular "lips" on the forward part of their thruster tunnels. See photo attached.

Has anyone seen any data on their effect?

Are they available for retrofit or are they built-up in GRP?

This concerns a 185mm tunnel on a 40' motorsailor.

Thanks - Henrik

 

They are usually built of the same material as hull. No data, but obviously having a big hole of the side of boat is a big drag. I have seen everything from covers, to retractable pods to all kinds of all looking holes. The best on a planning boat is simply place it just below waterline so when boat gets on plane hole is out of water.

The lip push water out so it doesn't crash onto back wall of bow tube. In the oval design the back of oval is more recessed to achieve the same thing.

Anyway installing a bow thruster is more complicated than opening a hole and filling around it. Structurally and hydrodynamically it can make substantial alteration to your boat. Remember the lost buoyancy also.

 

Modifying the inlet and outlet shape also affects the performance of the thruster. For best thruster performance, the inlet should have well rounded corners. This mod seems to make that worse which may reduce thrust. I suspect as with all things in boat design, this is a tradeoff of thruster performance for ahead running performance.

 

A thruster works for few just minutes during each trip - so, when a choice has to be made, the precedence is obviously given to solutions which increase the overall fuel efficiency of the vessel (which means a reduction of drag), not to the ones which increase of thruster performance.

 

That looks more like a fabrication artifact than any planned hydrodynamic effect. I have an idea what they are trying to do (see Hoerner, FDD, chpt 9, sect E), and maybe they have some pretty CFD plots that support that, but h is not large enough relative to D and Rn to make any difference to the opening vortex drag at motorsailer speeds in a real seaway. There might be some small anti-starvation effects, but those would be lost in the negative effects of the inlet asymmetry.

 

I think it would be very interesting to do CFD analyses on this contraption. Does anyone have an idea who could do that and an estimated cost?

 

The analysis can be done on any modern PC with CFD software. The cost is related to man-hours required to model the problem, run the iterations and analyse the results.

Then you can either accept them as unvalidated rough guidelines or can try to validate them experimentally, in order to get a certainty that you're not just looking at some fancy colored graphs but at a really useful stuff. In this latter case the cost would really soar.

 

Thanks Daiquiri

I already have had a civil engineer with cfd competence look at it, but his knowledge of boats is so limited that it was a waste of time. Not money though....

 

I might give it a try this weekend, for recreational purposes.

 

I used to be involved with hydraulic bow thrusters for larger craft and when the RNLI looked into bow thrusters and particularly the tunnel design and its effect on the boat's performance, I remember they concluded that fairing of the leading edge of the tunnel, but rather more smoothly than your photograph shows, was worth doing, but nothing else. I doubt much has changed over the last 10 years or so.
You might like to look at this link:
http://en.wikipedia.org/wiki/File:Relief_Lifeboat_The_Will_out_of_the_water.jpg

 

I have not heard them called "lips" before. A few months ago I was looking at a newly installed tunnel and the guy I was talking to called it an "eyebrow" which I thought was a good description.

I don't have them on my tunnel and one thing I have noticed is that if I apply thrust when I am moving slowly ahead I get much more sideways swing than when I am not moving.

 

You are quite right I have found. The official term is indeed 'eyebrows'. It also appears that a divice at the aft end of the tunnel is referred to as a 'tung'.